Syria is facing concerted international calls to allow United Nations humanitarian chief Valerie Amos access to the country as rebels retreated from a key stronghold.

Russia and China backed a UN statement criticising the regime's refusal to allow her to inspect the bloody aftermath of 11 months of violent repression.

But the two countries, which vetoed an Arab-backed peace plan last month, continued to resist pressure to join global condemnation of human rights violations.

Rebel forces last night made what they described as a "tactical retreat" from the besieged Baba Amr area of the city of Homs, which has become emblematic of the bitter struggle.

As government forces took back control, and amid warnings of brutal reprisals, president Bashar Assad's regime indicated that it would allow access to the International Committee of the Red Cross.

In a rare moment of agreement across the UN Security Council however, it issued a statement calling for Baroness Amos and humanitarian personnel also to be granted "immediate and unhindered access".

More than 7,500 civilians have been killed so far in the crackdown, the UN estimates, and its top human rights body voted to condemn Syria for "widespread and systematic violations" though without Russian or Chinese support.

Russian prime minister Vladimir Putin, who is seeking a return to the presidency in elections this weekend, denied Moscow had any special relationship with Damascus however and said the Syrians must choose who governs them.

In an interview with The Times, he said: "When Bashar al-Assad came to power he visited London and other European capitals first. We don't have a special relationship with Syria.

"It is up to the Syrians to decide who should run their country, We need to make sure they stop killing each other."

The withdrawal of rebels from Baba Amr came as the Foreign Office investigated the authenticity of videos posted by activists in Syria apparently showing Sunday Times reporter Marie Colvin and French photographer Remi Ochlik being buried.

A man in the videos says the pair, killed last week in a rocket attack on the besieged Baba Amr district of Homs, were interred in a local cemetery because power shortages meant their bodies could not be preserved.

"We haven't been able to get any confirmation that the video claiming to show Marie Colvin being buried is real," a UK government spokeswoman said.

Two French journalists, Edith Bouvier and William Daniels, have escaped to Lebanon after being trapped in Baba Amr, French president Nicolas Sarkozy said.

Prime Minister David Cameron spoke of his frustration at Britain's inability to do more directly to to prevent the slaughter in Syria - and said the UK was gathering evidence for a prosecution of Assad.

"It is extremely frustrating that we cannot do more," he said. "The scenes that we have seen on our televisions are appalling.

"I wish we could do more but we have to be realistic about what we can achieve about holding them to account. Gathering the evidence, using that evidence to build a case in international law that he can never hide from - that we can do."

The rapidly deteriorating security and humanitarian situation yesterday led the UK to announce that it had withdrawn its diplomatic staff from Syria and closed the Damascus embassy.

Foreign Secretary William Hague, announcing the decision to MPs, urged those fighting for the Assad regime to lay down their arms.